1. Field of the Invention
The present invention relates to a fixation structure for a valve system rotation shaft of an internal combustion engine.
2. Description of Background Art
A fixation structure for a valve system rotation shaft for an internal combustion engine is disclosed, for instance, in JP-A No. 2000-170506. This fixation structure rotatably fixes a camshaft for driving an engine valve mounted within a cylinder head of the internal combustion engine. The engine valve is mounted relative to the cylinder head through the use of a lower cam holder and a cam cap wherein the lower cam holder is fixed and is formed separately from the cylinder head. A holder fixation section for securing the lower cam holder and a cover connection section for connecting a base end of an external wall of a head cover at the peripheral side of the holder fixation section are positioned substantially flush with each other and mounted on the side toward the head cover of the cylinder head. The lower cam holder is mounted so that it is projected from the holder fixation section and cover connection section toward the head cover. Further, the external wall of the head cover extends toward the cylinder head in such a manner so as to cover the circumference of the lower cam holder.
However, when the external wall of the head cover is extended to increase its depth as in the above related-art configuration, the head cover is likely to vibrate, thereby causing resonance. Therefore, it is necessary to assure adequate rigidity by increasing the thickness or providing an additional reinforcement or take other vibration/noise control measures. This is likely to increase the cost, size, and weight of the head cover. Further, the use of the related-art configuration increases the depth of the head cover. Therefore, if there is no unoccupied space above the head cover, the head cover cannot be removed or reinstalled with the engine mounted in a vehicle. This situation is disadvantageous in that the engine needs to be demounted when, for instance, light-duty maintenance tasks such as tappet clearance adjustments are to be performed.